Add test showing bug

Author: owen-mc

go/ql/test/query-tests/RedundantCode/UnreachableStatement/CONSISTENCY/CfgConsistency.expected

@@ -1,5 +1,5 @@
 consistencyOverview
-| deadEnd | 9 |
+| deadEnd | 10 |
 deadEnd
 | main.go:17:2:17:10 | select statement |
 | main.go:109:2:109:10 | select statement |
@@ -10,3 +10,4 @@ deadEnd
 | main.go:145:2:145:10 | select statement |
 | main.go:151:2:151:10 | select statement |
 | main.go:157:2:157:10 | select statement |
+| main.go:163:2:163:10 | select statement |

go/ql/test/query-tests/RedundantCode/UnreachableStatement/main.go

@@ -159,4 +159,10 @@ func test19() mystruct {
 	return mystruct{test10(1), test10(2) == 2} // $ Alert
 }
 
+func test20() {
+	select {}
+	panic("unreachable") // OK: allowlisted statement
+	unreachable()        // $ MISSING: Alert
+}
+
 func main() {}